Update Node.js to v6.17.1

Author: mend-5034428[bot]

.travis.yml

@@ -1,7 +1,7 @@
 language: node_js
 node_js:
-  - "6.0"
-  - "6.9"
+  - "6.17.1"
+  - "6.17.1"
   - "8.0"
   - "8"
   - "10"